I was booked on an evening IAD-SFO and had used GGUs to upgrade me and Mrs. Mumphrey to F. The flight was later cancelled and I was auto-rebooked to a morning IAD-SFO, approximately 12 hours earlier. That didn't work for us, so I called to rebook to the evening DCA-SFO (leaving at roughly the same time as my original flight), and to my surprise the agent kept us in F (moving over the GGUs), despite there not being any U space. I had the exact opposite experience the last time I needed to make a change to a flight with GGUs after a major schedule change.